1. Field of the Invention
The present invention relates to plasma processing apparatus and method of processing substrates such as semiconductor wafers under plasma.
2. Description of the Related Art
Each circuit wiring in highly integrated semiconductor devices such as 16M and 64-MDRAM has had a half or quarter micron order diameter these days. The decompressed CVD, sputtering, etching, ashing apparatus and others are used in this super micro-process for circuit wirings. In the reactive ion etching (RIE) apparatus, for example, silicon dioxide (SiO.sub.2) film 3 on a silicon wafer W is partly removed by anisotropic etching (or high aspect ratio etching) to form a contact hole 4 having vertical side wall 5 shown in FIG. 3.
During the anisotropic etching process, however, chemically-active radicals (or active species) are generated together with reactive ions in plasma. These active radicals react with SiO.sub.2 to thereby cause SiO.sub.2 film 3 to be isotropy-etched. As the result, the contact hole 4 has a bowing-shaped side wall 5 as shown in FIG. 1 or taper-shaped one 5 as shown in FIG. 2. To solve this problem, therefore, the wafer on a suscepter is cooled to be less influenced by plasma radiation heat. The generation of active radicals can be thus suppressed.
On the other hand, the wafer surface is needed to be as uniform as possible in temperature to attain a uniform etching rate during the etching process. It is difficult, however, to completely and tightly contact the wafer with the suscepter (or lower electrode) all over it, and the wafer is not fully cooled only by heat transmission attained between solid matters contacted each other. This incomplete cooling of the wafer is caused in the plasma CVD and ashing apparatus as well. In these apparatus, the process chamber is exhausted vacuum from several hundreds Torr to several mill Torr. The wafer is thus vacuum-insulated from the suscepter and heat exchange between them becomes incomplete. In order to solve this, therefore, gases are introduced into a clearance between them to develop heat exchange between them by heat exchange gases so as to quickly and uniformly cool the wafer.
As shown in FIG. 4, however, the center area of the wafer has a substantially uniform temperature distribution but the temperature distribution is not uniform at the peripheral area of the wafer. The peripheral area of the wafer is usually higher in temperature than the center area thereof. As shown in FIGS. 1 and 2, therefore, anisotropic etching is conducted at the edge area of the wafer to thereby provide contact holes each having a side wall not excellent in its verticality.
Further, the suscepter of the conventional apparatus has an outer diameter smaller than that of the wafer not to be damaged by plasma radiation heat. That area of the wafer which is projected outwards from the outer rim of the suscepter can be less cooled in this apparatus. This causes the etching rate not to be uniform in the whole area of the wafer.